Tension members, such as prestressing tendons, are used to overcome concrete's natural weakness in tension. The method of prestressing concrete is used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. This method has also been extended to large civil work structures like tanks, dams and nuclear containments. Traditional reinforced concrete is based on the use of steel reinforcement bars (rebars) inside poured concrete. Prestressing tendons, generally composed of tensile cables made of high strength steel strands or rods, are used to provide a clamping force which produces a compressive stress on the concrete member to offset the tensile stress that the concrete member would otherwise experience due to an applied load.
The prestressing tendons are generally made up of a plurality of wires, bars or strands, the strands being further made up of several twisted metal wires. Known strands used in prestressing tendons are generally made up of metallic wires, for example steel wires. In some applications these wires are twisted together, and are coated with a protective filler and wrapped in a protective sheath of polymeric material, which may be extruded around the bundle of twisted-together wires.
Prestressed concrete can generally be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.
Prestressed concrete by pretensioning is obtained by casting concrete around already tensioned tendons. This method produces a good bond between the concrete and tendon, with concrete protecting the tendon from corrosion and allowing for direct transfer of tension. The cured concrete can then adhere and bond to the tendons, and when the tension is released, the compressive stress is transferred to the concrete by bond. However, this method requires stout anchoring points between which the tendon is to be stretched, and the tendons are usually in a straight line. No ducts are needed for the tendons.
Prestressed concrete by applying the method of bonded post-tensioned concrete comprises applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic or steel duct (often curved). In follow the area where otherwise tension would occur in the concrete element. A set of tendons is fed through the duct, and the concrete is poured. The tendons may also be fed after pouring the concrete. Once the concrete has hardened, the tendons are tensioned by e.g. hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications, they are wedged in position so that the tension is maintained after the jacks are removed and the pressure is transferred to the concrete through the anchoring elements. Finally the duct is then filled with a hardening protective filler such as grout to protect the tendons from corrosion and to provide bond. This method is commonly used to create monolithic slabs for building construction and in the construction of various types of bridges.
Unbonded post-tensioned concrete differs from bonded post-tensioning by providing tendons with permanent freedom of movement relative to the concrete. To achieve this, according to one solution each individual tendon or strand is coated with a layer of grease (usually lithium-based) and covered by a plastic sheathing formed in an extrusion process. These coated and sheathed tendons are either placed directly inside the concrete or alternatively inside a duct which is finally filled with a hardening protective filler such as grout. Alternatively, non-coated and non-sheathed tendons (same as for bonded post-tensioned concrete above) may be installed inside the duct which then may be filled with a flexible protective filler such as grease or wax to prevent bond.
In post-tensioning methods, difficulties often arise when feeding tension members into a duct. The feeding operation is generally done by feeding devices specifically designed for this purpose. When tension members are individually fed into a duct, then they are generally pushed by the feeding device, also known as a strand pusher. The ducts can be very long and curved. Especially in these situations the tension member can get blocked inside the duct. This can be very problematic, especially if there is a protective sheathing around the tension member. In this case, the feeding device can damage the protective sheathing when trying to push the blocked tension member further into the duct. Tension members with a damaged sheathing are prone to corrosion before the filling of the duct with a protective filler is completed. Furthermore, the damaged sheathing may make it impossible to replace these tension members later. If the protective sheathing gets damaged, then often the whole tension member feeding operation has to be started again with a tension member having an undamaged protective sheathing.
It is the object of the present invention to overcome the problems identified above related to the feeding of tension members into ducts.